Nondestructive testing (NDT) is a discipline which aims at evaluating the integrity and/or physical properties of materials without permanently altering the object being inspected. There are numerous NDT methods, tailored to various material, component and defect types. Pulsed Eddy Current (PEC) or Transient Eddy Current (TEC) techniques are especially efficient at measuring thickness variations in insulated and/or thick conductive objects. The working principle is to induce transient eddy currents inside the object being inspected by means of a sharp electromagnetic transition and measure its decay to infer material thickness or property changes.
The intensity of the produced magnetic field is proportional to the turn density of the coil and the current passing through it. However, as turn density increases, so does the inductance of the coil. Hence, in most PEC systems, the number of turns in the transmitting coil is kept to a minimum to reduce the inductance of the coil and therefore provide a sudden and/or drastic cutting-off of the coil. Using low inductance transmitting coil ensures that one can measure the magnetic field decay coming from the part without being overly affected by the transmitter coil itself. However, with such a design, generating a sufficiently strong magnetic field requires a very high current, which is not always practical. For example, when the transmitting coil is far from the acquisition instrument and an extension cable is required, significant power losses will ensue in the cable from the Joule effect. Such losses represent a major limitation, especially for a battery-operated instrument.
In contrast, the receiver coil requires very different characteristics to maximize its effectiveness. Its sensitivity will be directly proportional to the number of turns and will not be negatively impacted by the resulting high inductance. This stark difference between the optimal turn density for the transmitting and receiver coils encourages usage of separate sets of coils to prevent a prohibitive loss of performance in prior art systems.
In the context of a compact and portable NDT probe, this represents a major drawback as it increases the amount of components and the footprint they occupy. Additionally, some valuable insight can be gained from measuring the eddy current-produced magnetic field at the exact same position/orientation as the point of transmission. This can prove challenging when using an independent transmitter/receiver system due to potential crosstalk between the two sets of coils. Another downside of having independent sets of transmitters and receivers is that it increases the number of electrical conductors needed to communicate between the probe and the instrument, thus increasing the weight and lowering the flexibility of the electrical cable.